In the second half of the 19th century, physical views on the nature of the propagation of light, the action of gravity and some other phenomena more and more clearly began to encounter difficulties. They were connected with the ethereal concept dominating in science. The idea of conducting an experiment that would resolve the accumulated contradictions, as they say, was in the air.
In the 1880s, a series of experiments was set up, very complex and subtle for those times - Michelson's experiments to study the dependence of the speed of light on the direction of motion of the observer. Before dwelling in more detail on the description and results of these famous experiments, it is necessary to recall what the concept of aether was and how the physics of light was understood.
19th century views on the nature of the world
At the beginning of the century, the wave theory of light triumphed, receiving brilliant experimentalconfirmation in the works of Jung and Fresnel, and later - and theoretical justification in the work of Maxwell. Light absolutely undeniably exhibited wave properties, and the corpuscular theory was buried under a pile of facts that it could not explain (it would only be revived at the beginning of the 20th century on a completely new basis).
However, the physics of that era could not imagine the propagation of a wave otherwise than through the mechanical vibrations of any medium. If light is a wave, and it is able to propagate in a vacuum, then scientists had no choice but to assume that the vacuum is filled with a certain substance, due to its vibrations conducting light waves.
Luminous Aether
The mysterious substance, weightless, invisible, not registered by any devices, was called ether. Michelson's experiment was just designed to confirm the fact of its interaction with other physical objects.
Hypotheses about the existence of ethereal matter were expressed by Descartes and Huygens in the 17th century, but it became necessary as air in the 19th century, and at the same time led to insoluble paradoxes. The fact is that in order to exist in general, the ether had to have mutually exclusive or, in general, physically unreal qualities.
Ether concept contradictions
To match the picture of the observed world, the luminiferous ether must be absolutely motionless - otherwise this picture would be constantly distorted. But his immobility was in irreconcilable conflict with Maxwell's equations and the principleGalilean relativity. For the sake of their preservation, it was necessary to admit that the ether is carried away by moving bodies.
Besides, ethereal matter was thought to be absolutely solid, continuous and at the same time in no way hindering the movement of bodies through it, incompressible and, moreover, possessing transverse elasticity, otherwise it would not conduct electromagnetic waves. In addition, the ether was conceived as an all-pervading substance, which, again, does not fit well with the idea of his passion.
The idea and the first production of Michelson's experiment
American physicist Albert Michelson became interested in the aether problem after reading Maxwell's letter, published after Maxwell's death in 1879, describing an unsuccessful attempt to detect the Earth's motion with respect to the aether in the journal Nature.
In 1881, Michelson's first experiment took place to determine the speed of light propagating in different directions relative to the ether, an observer moving with the Earth.
The Earth, moving in orbit, must be subjected to the action of the so-called ethereal wind - a phenomenon similar to the flow of air running on a moving body. A monochromatic light beam directed parallel to this “wind” will move towards it, losing a little in speed, and vice versa (reflecting from the mirror) in the opposite direction. The change in speed in both cases is the same, but it is achieved in different times: the slowed down "oncoming" beam will take longer to travel. So the light signalemitted parallel to the "ether wind" will necessarily be delayed relative to a signal traveling the same distance, also with reflection from the mirror, but in a perpendicular direction.
To register this delay, a device invented by Michelson himself was used - an interferometer, whose operation is based on the phenomenon of superposition of coherent light waves. If one of the waves was delayed, the interference pattern would shift due to the resulting phase difference.
Michelson's first experiment with mirrors and an interferometer did not give an unambiguous result due to insufficient sensitivity of the device and underestimation of numerous interferences (vibrations) and caused criticism. Significant improvement in accuracy was required.
Repeated experience
In 1887, the scientist repeated the experiment together with his compatriot Edward Morley. They used an advanced setup and took special care to eliminate the influence of side factors.
The essence of the experience has not changed. The light beam collected by means of a lens was incident on a semitransparent mirror set at an angle of 45°. Here he divided: one beam penetrated through the divider, the second went in a perpendicular direction. Each of the beams was then reflected by an ordinary flat mirror, returned to the beam splitter, and then partially hit the interferometer. The experimenters were confident in the existence of an "ethereal wind" and expected to get a completely measurable shift of more than a third of the interference fringe.
It was impossible to neglect the movement of the solar system in space, so the idea of the experiment included the ability to rotate the installation in order to fine-tune the direction of the "ethereal wind".
To avoid vibration interference and distortion of the picture when turning the device, the entire structure was placed on a massive stone slab with a wooden toroidal float floating in pure mercury. The foundation under the installation was buried to the rock.
Experimental results
Scientists conducted careful observations throughout the year, rotating the plate with the device clockwise and counterclockwise. The interference pattern was recorded in 16 directions. And, despite the accuracy unprecedented for his era, Michelson's experiment, conducted in collaboration with Morley, gave a negative result.
In-phase light waves leaving the beam splitter reached the finish line without phase shift. This was repeated every time, at any position of the interferometer, and meant that the speed of light in Michelson's experiment did not change under any circumstances.
Checking the results of the experiment was carried out repeatedly, including in the XX century, using laser interferometers and microwave resonators, reaching an accuracy of one ten billionth of the speed of light. The result of the experience remains unshakable: this value is unchanged.
The meaning of the experiment
From the experiments of Michelson and Morley it follows that the "ethereal wind", and, consequently, this elusive matter itself simply does not exist. If any physical object is fundamentally not detected in any processes, this is tantamount to its absence. Physicists, including the authors of the brilliantly staged experiment, did not immediately realize the collapse of the concept of the ether, and with it the absolute frame of reference.
Only Albert Einstein in 1905 managed to present a consistent and at the same time revolutionary new explanation of the results of the experiment. Considering these results as they are, without trying to draw speculative ether to them, Einstein arrived at two conclusions:
- No optical experiment can detect the rectilinear and uniform motion of the Earth (the right to consider it as such is given by the short duration of the act of observation).
- Regarding any inertial frame of reference, the speed of light in a vacuum is unchanged.
These conclusions (the first - in combination with the Galilean principle of relativity) served as the basis for Einstein's formulation of his famous postulates. So the Michelson-Morley experiment served as a solid empirical basis for the special theory of relativity.
